This invention is an improvement of the propeller fan disclosed in U.S. Pat. No. 2,985,245.
Fans according to the invention are used on wind machines for protection of citrus and avocado trees and other growth and crops against damage from cold or other climatic conditions. In some situations a disturbance of the ambient air by forced air flow from the wind machine will suffice to dissipate the harmful effect of quiescent pockets or blankets of cold air which settle over, around and under trees and plants. Under extreme conditions, heating of the circulated air is required.
The fans are rotated about 590 r.p.m. on the top of columns varying in height from about 10 ft. to 40 ft. The fans are driven either by electric motors or by internal combustion engines which may either be at the foot of the column or on the top or adjacent the top of the column. The fan drive assemblies at the top of the column are typically rotated horizontally through 360.degree. at the rate of about 41/2 minutes per complete rotation. The fan diameters vary between 11 ft. and 17 ft.
In propeller fans an impulse or thrust is produced by the action of the moving blades on the air impelled. The impulse is directly related to the change in energy travel of the air passing through the plane of the fan. In a fixed fan, the impulse must be restrained completely in order to prevent axial travel of the entire fan mechanism. Therefore, the aerodynamic forces on the blading must be balanced out by reacting forces in the blades and fan structure.
The fan structure providing the best aerodynamic efficiency is in the form of thin extruded aluminum airfoil, except for the structural properties required of the region where the propeller fan joins the hub. At these connections large moments may develop which produce bending stress that actually exceed the tension stresses caused by the centrifugal force. It is thus necessary to provide a mechanism which makes use of the centrifugal force to overcome or remove the bending moment. The connection between the hub and each blade must form a continuous elastic deflectable joint.
In the prior art, solid splice plates were bolted to both sides of the blades and to tangs extending outwardly from the hub to form hub and blade connections of high strength but with a bending stiffness that was relatively high. While this type of connection was substantially successful during the years that they were in use, there have been a number of failures which led to efforts to develop the improvement provided in the present invention. The number of failures was not so great to be an inordinate problem in and of themselves, but the principal problem was in the danger that was produced when a fan broke rotating at about 590 r.p.m. where personnel were working. The fan blades typically broke adjacent the outer end of the splice plates, or at the outer bolts holding the splice plates to the fan.